Diode and applications thereof

ABSTRACT

A diode with low substrate current leakage and suitable for BiCMOS process technology. A buried layer is formed on a semiconductor substrate. A connection region and well contact the buried layer. Isolation regions are adjacent to two sides of the buried layer, each deeper than the buried layer. The isolation regions and the buried layer isolate the connection zone and the well from the substrate. The first doped region in the well is a first electrode. The well and the connection region are electrically connected, acting as a second electrode.

BACKGROUND

The present invention relates in general to a diode providing electrostatic discharge (ESD) protection, and in particular to a diode manufactured by bipolar complementary metal oxide semiconductor (BiCMOS) processes.

Among ESD protection devices, a diode has one of the simplest structures. Properly forward biasing a diode during an ESD event requires only a small silicon area for effective protection. FIG. 1 is a cross section of a conventional diode formed with a heavy-doped P-type (P+) region and an N-type well (NW).

FIG. 2 shows a conventional diode string, connected between high voltage (V_(high)) and low voltage (V_(low)) power lines, serving as an ESD protection device. The conventional diode string consists of several diodes connected in series. As shown in FIG. 2, a parasitic Darlington amplifier is formed in the diode string by the series-connected parasitic PNP bipolar transistors (BJTs), resulting in substrate current leakage forward to the grounded P-type (P) substrate. This substrate current leakage becomes more severe when operating temperature or diode number in the diode string increases.

There are several solutions for substrate current leakage. FIG. 3 shows a conventional circuit schematic suppressing substrate current leakage, in which the bias circuit 8 conducts a small amount of forward current to a lower portion of a diode string. Experimental results from the circuit indicate that, at high temperature, substrate current leakage still occurs at significant magnitude.

Another solution involves modification of the structure of each diode. By lowering the common-base gain of each parasitic BJT, the overall current gain of a Darlington amplifier is decreased, reducing substrate current leakage. Alternatively, eliminating formation of a Darlington amplifier, the substrate current leakage of a diode string may be substantially diminished. FIG. 4 shows a diode string with diodes of an exemplary diode structure. In comparison with the diode of FIG. 1, each diode in FIG. 4 has not only a P+ region and an NW, but also a deep NW under the NW. The common-base current gain of each parasitic BJT is reduced by the enlarged base width thereof. The overall current gain of a Darlington amplifier is accordingly decreased, and the substrate current leakage is reduced.

FIG. 5 shows another diode string in which, for each diode, the P-type well (PW) is completely enclosed by an NW and a deep NW. Through a wire connection, the voltage potentials of the NW and the deep NW are the same as that of the anode of the enclosed diode. In FIG. 5, because the base and emitter of each parasitic PNP BJT are at the same voltage potential and the emitter, the PW, has much lower doped concentration, the common-base current gain of each parasitic PNP BJT is decreased, such that each parasitic PNP BJT is difficult to activate. Furthermore, the Darlington amplifier is not formed, due to the modification of the diode structure, improving substrate current leakage.

The diodes in FIGS. 4 and 5 are suitable for CMOS semiconductor processes providing formation of a deep NW. However, with regard to a diode string with ESD protection for SiGe BiCMOS processes, the diode string in FIG. 6 is generally utilized. In FIG. 6, each diode has a heavy-doped N-type (N+) buried layer 94. Because the emitter, the PW 90, has a much lower doping concentration, the common-base current gain of each parasitic PNP BJT is decreased. Furthermore, the Darlington amplifier is not formed, due to modification of the diode structure. Both results improve substrate current leakage.

SUMMARY

Embodiments of the present invention provide a diode with low substrate current leakage. The diode comprises a semiconductor substrate, a buried layer, a well, a connection region, two isolation regions, and a first doped region. The semiconductor substrate is of a first conductivity type. The buried layer is formed on the semiconductor substrate and of a second conductivity type opposite to the first conductivity type. The well is of the first conductivity type, formed on the buried layer. The connection region is of the second conductivity type, formed on the buried layer and contacting the buried layer. The two isolation regions are respectively formed at two sides of the buried layer, each deeper than the buried layer. The isolation regions, together with the buried layer, isolate the well and the connection region from the semiconductor substrate. The first doped region is of the second conductivity type and, on the well, acts as a first electrode of the diode. The connection region and the well are electrically connected to each other, acting as a second electrode of the diode.

The first conductivity type can be P-type and the second conductivity type N-type.

The connection region can be a sinker or another well.

Each isolation region can consist of a single deep trench or one deep trench and a shallow trench.

On the surfaces of the connection region and the well, second and third doped regions can be formed as electrical contacts.

The second and third doped regions can be connected via a wire connection or a silicide layer on the surfaces thereof.

A shallow trench or dummy gate can be formed between each two doped regions for electrical isolation.

The invention also provides an ESD protection circuit with a diode string. The ESD protection circuit can be employed in an input/output (I/O) port or between two power rails.

The voltage potential of a buried layer is the same as the anode of a diode on the buried layer, resulting in a parasitic BJT with a base and emitter of the same voltage potential. This parasitic BJT is difficult to activate during normal operation and has a lower common-base current gain, and, there no Darlington amplifier is formed in a diode string utilizing the diode of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative.

FIG. 1 is a cross section of a conventional diode formed with a heavy-doped P-type region and an N-type well;

FIG. 2 shows a conventional diode string;

FIG. 3 shows a conventional circuit schematic;

FIG. 4 shows a diode string with diodes of an exemplary diode structure;

FIG. 5 shows another diode string with diodes of an exemplary diode structure;

FIG. 6 shows a conventional diode string applicable in SiGe BiCMOS processes;

FIG. 7 shows a diode according to embodiments of the present invention;

FIGS. 8 and 9 show two other diodes according to embodiments of the present invention;

FIG. 10 shows a diode string 50 utilizing the diode in FIG. 7;

FIGS. 11 and 12 show two diode strings utilizing the diodes in FIGS. 8 and 9, respectively;

FIGS. 13A and 13B are two symbols to respectively refer to the diode and the diode string according to the present;

FIG. 14 illustrates an ESD protection circuit for an input/output port according to embodiments of the present invention;

FIG. 15 illustrates another ESD protection circuit for an input/output port according to embodiments of the present invention;

FIG. 16 illustrates a power rail clamping circuit according to embodiments of the present invention;

FIG. 17 illustrates another power rail clamping circuit according to embodiments of the present invention, utilizing substrate triggering; and

FIG. 18 is a cross section of a wafer utilizing the power rail clamping circuit of FIG. 17.

DESCRIPTION

FIG. 7 shows a diode according to embodiments of the present invention, manufactured by bipolar complementary metal oxide semiconductor (BiCMOS) processes. Diode 10 comprises a P substrate 14, an N+ buried layer 12, a P well (PW) 16, an N+ sinker 18, N+ regions 24 and 28, a P+ region 26, two deep trenches 20 and several shallow trenches 22˜.

N+ buried layer 12 is disposed on the P substrate 14. PW 16 and N+ sinker 18 are disposed on N+ buried layer 12 and generally formed in an epitaxy silicon layer of a BiCMOS process.

N+ region 24 serves as an electrical contact for N+ sinker 24, and P+ region 26 as an electrical contact for PW 16. On each surface of N+ regions 24 and 28, and P+ region 26, a silicide layer 32 enhances conductivity of each region. Through a wire connection 30, N+ region 24 and P+ region 26 are electrically connected to each other and to other circuits. PW 16, N+ sinker 18 and N+ buried layer 12 thus have the same voltage potential. Wire connections 30 generally consist of contacts, metal lines, vias and the like. Similarly, N+ region 28 can be connected to other circuits through another wire connection 30.

N+ region 28 is formed in PW 16, simultaneously forming a PN junction therein. N+ region 28 thus acts as a cathode of a diode, and PW 16 or its electrical contact, P+ region 26, as an anode of the diode.

In FIG. 7, an isolation region exemplarily comprises a shallow trench 22 a and a deep trench 20, both of isolation material, such as silicon dioxide. Each deep trench 20 is deeper than N+ buried layer 12. The cross section in FIG. 7 shows deep trenches 20 respectively contacting two sides of N+ buried layer 12. A top view of the diode in FIG. 7, however, may show that the two deep trenches 20 are in the same isolation zone enclosing the entire N+ buried layer 12.

These two isolation regions, each comprising a deep trench 20 and a shallow trench 22 a, together with N+ buried layer 12, electrically isolate PW 16 from P substrate 14. Shallow trench 22 c isolates P+ region 26 from N+ region 28, and shallow trench 22 b isolates P+ region 26 from N+ region 24.

In related art FIG. 6, N+ buried layer 94 is not connected to PW 90, such that its potential should be lower than that of PW 90. This voltage difference, about that to turn on a PN junction, eases to turn on the parasitic PNP BJT, resulting in significant substrate current leakage. Nonetheless, in FIG. 7, since PW 16 and N+ buried layer 12 have the same voltage potential, no voltage difference is induced between the emitter and the base of the parasitic PNP BJT, consisting of PW 16, N+ buried layer 12 and P substrate 14. This parasitic PNP BJT is difficult to activate, in addition to much smaller the common-base current gain, causing little substrate current leakage.

The most significant differences between an N+ buried layer of a BiCMOS process and a deep NW of a CMOS process are the functions provided. Generally, in a CMOS circuit, a deep NW provides isolation of a PW and a P substrate. In a BiCMOS circuit, in addition to isolation between a PW and a P substrate, an N+ buried layer usually acts a sub-collector, connecting the collector of a BJT. Accordingly, an N+ burier layer requires low resistance and is heavily doped to reduce sheet resistance. A deep NW, in order to provide adequate isolation, is generally lightly doped to enlarge the junction breakdown voltage between itself and a P substrate. As an example, for a 0.18 um BiCMOS process, the dosage concentration of an N+ burier layer is about 10¹⁵/cm²˜10¹⁷/cm²; and for a comparable 0.18 um CMOS process, the dosage concentration of an deep NW is about 10¹³/cm²˜10¹⁵/cm².

N+ sinker 18 in FIG. 7 can be replaced by a NW with lighter dosage concentration, capable to connect N+ buried layer 12 and PW to the same voltage potential.

FIG. 8 shows another diode structure according to embodiments of the present invention. Unlike the two shallow trenches 22 c and 22 b shown in FIG. 7, which isolate N+ region 24, P+ region 26 and N+ region 28, in FIG. 8, they are respectively replaced by two dummy gates 40 a and 40 b. Each dummy gate has a polysilicon gate. During ion implantation to form N+ regions 24 and 28, a portion of the polysilicon gate adjacent to a N+ region is simultaneously doped into N-type doped polysilicon. During ion implantation to form P+ region 26, a portion of the polysilicon gate adjacent to a P+ region is simultaneously doped into P-type doped polysilicon, as shown in FIG. 8.

The shallow trench or dummy gate between P+ region 26 and N+ region 24 can be omitted to conserve space. FIG. 9 shows another diode according to embodiments of the present invention. Unlike FIG. 7, FIG. 9 lacks shallow trench 22 c. As shown in FIG. 9, a silicide layer 32 on surfaces of P+ region 26 and N+ region 24 shorts them. Hence, the wire connection 30 for P+ region 26 and N+ region 24 requires only a contact hole for necessary connection to the anode of a diode.

FIG. 10 shows a diode string 50 utilizing the diode in FIG. 7. In FIG. 10, the diode string 50 consists of 4 diodes forward connected in series, wherein an anode of a diode is connected to a cathode of another diode, whose anode is connected to a cathode of anther diode, such that, each diode has the same polarity. The diode string 50 thus acts as a compound diode having an activation voltage equal to the sum of all the activation voltages of the diodes therein, a major anode at the anode of the first diode in the diode string 50, and a major cathode at the cathode of the last diode in the diode string 50. While in FIG. 10, the number of the diodes is 4. The disclosure is not limited thereto. In application, the number of diodes forward connected in series is determined by, and proportional to, activation voltage required for the diode string.

Acting as an ESD protection device, the major anode and cathode of the diode string 50 are generally coupled to metal lines 52 and 54, respectively, with voltage potential of the metal line 52 normally exceeding that of the metal line 54. For example, when the metal line 52 is a high voltage power rail, the metal line 54 can be a low voltage power rail or an input/output (I/O) pad of an input/output port. When the metal line 54 is a low voltage power rail, the metal line 52 can be a high voltage power rail or an input/output pad of an input/output port.

FIGS. 11 and 12 show diode strings utilizing the diode structures in FIGS. 8 and 9, respectively, and are self-explanatory to a person skilled in the art.

There is no requirement that all the diodes in a diode string have the same diode structure. Each diode in a diode string can have an unique diode structure. For example, a diode of FIG. 7 can be forward connected in series with a diode of FIG. 9 to from a diode string.

FIGS. 13A and 13B are two symbols to respectively refer to the diode and the diode string according to the present invention. Hereinafter, symbol 60 in FIG. 13A represents a diode according the present invention and could be anyone of FIGS. 7, 8, and 9. Symbol 62 in FIG. 13B represents a diode string according to the present invention and has at least two diodes forward connected in series.

FIG. 14 illustrates an ESD protection circuit for an input/output port according to embodiments of the present invention. A diode D1 is connected between a power rail VDD and an I/O pad, and a diode D2 connected between an I/O pad 64 and a VSS power rail. During normal operation, both the diodes D1 and D2 are reverse biased. A diode string S0 is connected between the VDD and VSS power rails, with diodes therein forward biased during normal operation. When an ESD pulse relatively positive to the VSS power rail occurs at the I/O pad 64, ESD charges can be released through the I/O pad 64, the diode D1, the VDD power rail and the diode string S0, and to the VSS power rail, thereby protecting internal circuits from ESD damage. Since diodes D1 and D2 are forward biased to conduct ESD current, they can be small in size. This ESD protection circuit is especially suitable for radio frequency (RF) integrated circuits (ICs). As shown in FIG. 14, only two small diodes D1 and D2 are additionally connected to the I/O pad 64, adding little capacitance loading and significant diminishing influence of the high frequency response on the I/O port.

FIG. 15 illustrates another ESD protection circuit for an input/output port according to embodiments of the present invention. A diode D1 and a diode string S1 are connected in parallel between the VDD power rail and an I/O pad 64 while a diode D2 and a diode string S2 are connected in parallel between the VSS power rail and the I/O pad 64. During normal operation, both diodes D1 and D2 are reverse biased, both diode strings S1 and S2 are forward biased, and all diodes D1 and D2 and diode strings S1 and S2 are open circuits. When an ESD pulse relatively positive to the VSS power rail occurs at the I/O pad 64, ESD charges can be released through the forward biased diode string S2 and to the VSS power rail, thereby protecting internal circuits from ESD damage.

FIG. 16 illustrates a power rail clamping circuit according to the present invention. A diode D0 and a diode string S0 are connected in parallel between a V_(high) power rail and a V_(low) power rail. During normal operation, the diode D0 is reverse biased and the diode string S0 forward biased. When an ESD pulse relatively positive/negative to the V_(low) power rail occurs at the V_(high) power rail, ESD charges can be released through the forward biased diode string S0/diode D0 and to the VSS power rail, thereby protecting internal circuits from ESD damage.

FIG. 17 illustrates another power rail clamping circuit according to embodiments of the present invention, utilizing substrate triggering. FIG. 18 is a cross section of a wafer utilizing the power rail clamping circuit of FIG. 17. In FIG. 17, the primary discharge element is a field oxide MOS transistor FOD connected between VDD and VSS power rails. A diode string S3 and a resistor R_(sub) are connected in series between the two power rails. The connection node between the diode string S3 and the resister R_(sub) is also connected to the base of a parasitic NPN BJT under FOD. In FIG. 18, the resistor R_(sub) is a spread resistor of the substrate 14 disposed between the base of the NPN BJT and a P+ region 66, a contact of the P substrate 14.

During normal operation, activation voltage of the diode string S3 exceeds the voltage difference between the VDD and VSS power rails, and, as a result, the diode string S3 and the PNP BJT under FOD remain open. During an ESD event, activation of the diode string S3 conducts small ESD current through the resistor R_(sub), and when voltage drop cross the resistor R_(sub) is sufficient, the NPN BJT is activated, conducting large ESD current from the VDD power rail, through the parasitic NPN BJT, and to the VSS power rail.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art) Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

1. A diode characterized with low substrate current leakage, comprising: a semiconductor substrate of a first conductivity type; a buried layer on the semiconductor substrate, of a second conductivity type opposite to the first conductivity type; a well of the first conductivity type on the buried layer; a connection region of the second conductivity type, on the buried layer and contacting the buried layer; two isolation regions, respectively on two sides of the buried layer, each deeper than the buried layer, wherein the isolation regions together with the buried layer isolate the well and the connection region from the semiconductor substrate; and a first doped region of the second conductivity type in the well acting as a first electrode of the diode; wherein the connection region and the well are electrically connected, acting as a second electrode of the diode.
 2. The diode as claimed in claim 1, wherein each isolation region comprises: a deep trench; and a shallow trench stacked on and more shallow than the deep trench.
 3. The diode as claimed in claim 1, further comprising a second doped region of the first conductivity type in the well, isolated from the first doped region, and serving as an electrical contact of the well.
 4. The diode as claimed in claim 3, further comprising a shallow trench between the first and the second doped regions.
 5. The diode as claimed in claim 3, further comprising a dummy gate between the first and the second doped regions, the dummy gate comprising a conductive gate, a portion of which is the first conductivity type, and another portion of which is the second conductivity type.
 6. The diode as claimed in claim 3, further comprising a third doped region of the second conductivity type on the connection region, isolated from the first doped region, and serving as an electrical contact of the connection region and the buried layer.
 7. The diode as claimed in claim 6, further comprising a shallow trench between the third and second doped regions.
 8. The diode as claimed in claim 6, further comprising a dummy gate between the third and the second doped regions, the dummy gate comprising a conductive gate, a portion of which is the first conductivity type, and another portion of which is the second conductivity type.
 9. The diode as claimed in claim 6, wherein the second doped region directly contacts the third doped region.
 10. The diode as claimed in claim 1, wherein the connection region is a sinker.
 11. An ESD protection circuit, comprising: at least one diode with low substrate current leakage, comprising: a semiconductor substrate of a first conductivity type; a buried layer on the semiconductor substrate of a second conductivity type opposite to the first conductivity type; a well of the first conductivity type on the buried layer; a connection region of the second conductivity type on the buried layer and contacting the buried layer; two isolation regions, respectively on two sides of the buried layer, each having a bottom deeper than the buried layer, wherein the isolation regions together with the buried layer isolate the well and the connection region from the semiconductor substrate; and a first doped region of the second conductivity type on the well acting as a first electrode of the diode; wherein the connection region and the well are electrically connected, acting as a second electrode of the diode; a first wire connection, electrically coupled to the first electrode; and a second wire connection, electrically coupled to the second electrode; wherein the first or second wire connection is a first power rail.
 12. The ESD protection circuit as claimed in claim 11, wherein the other wire connection is a second power rail.
 13. The ESD protection circuit as claimed in claim 11, wherein the other wire connection is connected to an input/output pad.
 14. The ESD protection circuit as claimed in claim 11, further comprising a plurality of diodes forward connected in series to form a diode string with first and second major electrodes, the first and second wire connections respectively connected to the first and second major electrodes, and, during normal operation, all diodes are forward biased.
 15. The ESD protection circuit as claimed in claim 14, further comprising a second diode between the first and second wire connections and reverse biased during normal operation.
 16. The ESD protection circuit as claimed in claim 14, further comprising: a primary discharge element between the first and second wire connections, having a trigger node; and a resistor connected with the diode string through a connection node; wherein the resistor and the diode string are connected in series between the first and second wire connections, and the connection node is connected to the trigger node.
 17. The ESD protection circuit as claimed in claim 14, wherein the primary discharge element is a field oxide MOS transistor, the trigger node is a base of a parasitic BJT in the field oxide MOS transistor, and the resistor is a spread resistor of the semiconductor substrate connected between the base and a contact of the semiconductor substrate. 